Apparatus and method for adaptively modulating signal by using layered time-space detector used in MIMO system

ABSTRACT

An apparatus for adaptively modulating/demodulating signals in a multi-input multi-output (MIMO) system having a layered time-space architecture detector and a method thereof is disclosed. The apparatus includes: a bit and power allocation information calculator for deciding an equivalent channel gain in a reverse order of Vertical-Bell laboratories Space Time (V-BLAST) based on MIMO channel information feedbacked from a receiver and determining the number of bits and transmission power to be transmitted to each transmitting antenna by using the equivalent channel gain; and adaptive modulation means for modulating signal of each layer with corresponding modulation method based on the determined number of bits and transmitting power, controlling the transmitting power and transmitting the adaptively modulated signal through each transmitting antenna. The present invention can improve performance without increase of implementation complexity and easily expand to MIMO-OFDM system by adaptively modulating and demodulating signals in reverse order of conventional V-BLAST detection method.

FIELD OF THE INVENTION

The present invention relates to an apparatus formodulating/demodulating signal by using a layered time-spacearchitecture detector in a multi-input multi-output (MIMO) system and amethod thereof; and, more particularly, to the apparatus formodulating/demodulating signal by using a layered time-spacearchitecture detector in a multi-input multi-output (MIMO) system inorder to increase a system performance by deciding an equivalent channelgain, deciding the number of bits for transmitting through each antennaand deciding a transmission power by using the a greedy algorithm basedon the decided equivalent channel gain.

DESCRIPTION OF RELATED ARTS

Generally, a multi-input multi-output (MIMO) system is a wirelesscommunication system obtaining high frequency efficiency by transmittingeach different data through a plurality of transmission antennas in anidentical bandwidth. There have been several detection methods for MIMOsystems, such as a Diagonal Bell Laboratories Space Time (D-BLAST)system proposed in an article by G. J. Foschini, entitled “LayeredSpace-Time Architecture for Wireless Communication in a FadingEnvironment When using Multi-Element Antennas,” Bell Labs TechnicalJournal, fall, pp. 41˜59, 1999 and a Vertical-Bell Laboratories SpaceTime (V-BLAST) system introduced in an article by P. W. Wolnianskyet.al., entitled “V-Blast: An Architecture for Realizing Very High DataRates Over the Rich-Scattering Wireless Channel.” Proc. InternationalSymposium Signals, Systems and Electronics, September, 1998.

The V-BLAST system is a modified scheme of the D-BLAST system.

Operations of detection in the V-BLAST system are explained in moredetail hereinafter. At first, a symbol corresponding to a layer havingthe largest equivalent channel gain is detected based on a MIMO channelmatrix and new channel matrix is built by nulling a channelcorresponding to the currently detected layer after canceling the effectof the detected symbol in the channel matrix H. And the above mentionedsteps are repeated until all symbols are detected. As a result of theseoperations, a layer having largest equivalent channel gain is detectedfor the first time. When the next symbol is detected, the effect of thefirst symbol is eliminated because it is considered as interference andthen the next symbol is detected. Therefore, a diversity gain can beobtained when the next symbol is detected and a performance isincreased.

In a meantime, a structure of V-BLAST system is in U.S. Pat. No.6,317,466 B1 issued to G. J. Foschini, entitled “Wireless CommunicationsSystem Having a Space-Time Architecture Employing Multi-Element antennaat both the Transmitter and Receiver” and also an adaptive modulationmethod is disclosed. But in the above mentioned patent by G. J.Foschini, a method for minimizing transmission power and changingmodulation method commonly used for all antennas.

Furthermore, a greedy algorithm for allocating power and the number ofbits in multicarrier systems is introduced in an article by C. Y. Wong,entitled “Multi-user OFDM with adaptive sub-carrier, bit and powerallocation”, IEEE Journal on Selected Areas in Communication, Vol. 17,pp. 1747˜1758, October, 1999. The greedy algorithm is used forcalculating a power and the number of bits of each subcarrier in orderto transmit all information bits with the minimum transmission powerwhile satisfying desired bit error rate under conditions such as singleuser orthogonal frequency division multiplexing (OFDM) is used, afrequency domain channel response corresponding to each subcarrier isknown at the transmitter, the number of bits for transmitting at oneOFDM symbol is predetermined and there is a desired bit error rate.

There are two methods implementing the MIMO-OFDM system with theadaptive modulation method. A first method is introduced in an articleby Ka-Wai Ng. et. al., entitled “A simplified bit allocation for V-BLASTbased OFDM MIMO system in frequency selective fading channels” IEEEinternational conference on communication, pp. 411˜415, 2002. A secondmethod is disclosed in an article by Ka-wai Ng et. al., entitled“Iterative bit & power allocation for V-BLAST based OFDM MIMO system infrequency selective fading channel”, Proc. Wireless Communications andNetworking Conference, 2002, pp. 271˜275.

The first method calculates the equivalent channel gain based on anordering method of the V-BLAST and decides the number of bits accordingto the greedy algorithm based on the calculated equivalent channel. Itis also disclosed that a method of selecting a subcarrier andtransmitting antenna in order to reduce the amount of informationtransmitted to the transmitter. The above mentioned approaches using theV-BLAST ordering method may obtain nearly same performance for alllayers when fixed modulation method is used. However, the performance isdegraded in a case of an adaptive modulation is used.

The second method allocates a predetermined number of bits to allpossible combinations of decision order for the optimal solution andselects the optimal order with minimum total transmission power.However, by reducing the number of combinations to be considered forfinding the optimal solution, the performance is degraded.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide anapparatus for adaptively modulating/demodulating signal by using alayered time-space architecture detector in a multi-input multi-output(MIMO) system for improving a system performance by deciding anequivalent channel gain, deciding the number of bits for transmittingthrough each antenna and a transmission power by using a greedyalgorithm based on the decided equivalent channel gain.

In accordance with an aspect of the present invention, there is providedan apparatus for adaptively modulating signal in a MIMO system having alayered space-time architecture based detector, the apparatus including:a bit and power allocation information calculator for deciding anequivalent channel gain-in a reverse order of Vertical-Bell laboratoriesSpace Time (V-BLAST) based on MIMO channel information feedbacked from areceiver and determining the number of bits and transmission power to betransmitted from each transmitting antenna by using the equivalentchannel gain; and adaptive modulation means for modulating signal ofeach layer with corresponding modulation method based on the determinednumber of bits and transmitting power.

In accordance with an aspect of the present invention, there is providedan apparatus for adaptively demodulating signal in a MIMO system havinga layered space-time architecture based detector, the apparatusincluding: MIMO channel estimation unit for estimating MIMO channel froma signal received through each receiving antenna; a bit and powerallocation information calculator for determining an equivalent channelgain in reverse order of Vertical-Bell laboratories Space Time V-BLASTbased on MIMO channel information from the MIMO channel estimation unitand determining the number of bits corresponding to each transmittingantenna by using the equivalent channel gain.

In accordance with an aspect of the present invention, there is providedapparatus for adaptively modulating and demodulating signals in MIMOsystem using multiple antennas at transmitter and receiver, theapparatus including: an adaptive modulation unit for adaptivelymodulating signals in order to transmit the modulated signal afterdetermining an equivalent channel gain in a reverse order of avertical-bell laboratories space time and determining the number of bitsand transmitting power based on the determined equivalent channel gain;and adaptive demodulation unit for detecting and adaptively demodulatingreceived signals through each receiving antenna in reverse order ofV-BLAST.

In accordance with an aspect of the present invention, there is provideda method for adaptively modulating signals in a MIMO system usingmultiple antennas in a receiver and transmitter, the method, includingthe steps of: a) determining equivalent channel gain in a reverse orderof V-BLAST at transmitter based on channel information feed backed fromthe receivers; and b) adaptively modulating signals by determining thenumber of bits and corresponding transmitting power to be transmittedthrough each layer (antenna) by using the equivalent channel gain in agreedy algorithm.

In accordance with an aspect of the present invention, there is provideda method for adaptively demodulating in MIMO systems, the methodincluding the steps of: a) estimating a channel from a signal receivedat each receiving antenna; b) deciding an equivalent channel gain in areverse order of a vertical-bell laboratories space time (V-BLAST) basedon the channel information; and c) detecting and adaptively demodulatingthe received signal using the adaptive modulation information for eachlayer.

In accordance with an aspect of the present invention, there is provideda computer readable recoding medium storing instructions for executing amethod for an adaptive modulation, the method including the steps of: a)at a transmitter, deciding an equivalent channel gain in a reverse orderof V-BLAST based on a feedback information from a receiver; and b) atthe transmitter, deciding the number of bit transmitting through eachlayer (transmitting antenna) and transmitting power based on theequivalent channel gain computed at the step a).

In accordance with an aspect of the present invention, there is provideda computer readable recoding medium storing instructions for executing amethod for an adaptive demodulation, the method including the steps of:a) estimating a channel from a signal received from each receivingantenna; b) deciding an equivalent channel gain in a reverse order ofV-BLAST based on the channel information; and c) detecting andadaptively demodulating by deciding the number bits for each layer.

BRIEF DESCRIPTION OF THE DRAWING(S)

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram for illustrating a MIMO wireless communicationsystem having a layered space-time architecture detector in accordancewith a preferred embodiment of the present invention;

FIG. 2 is a view showing changing of equivalent channel gain accordingto the detection order in layered space-time architecture in accordancewith the present invention;

FIG. 3 is a flowchart for explaining a method for determining theequivalent channel gain in a reverse order of the V-BLAST detectionmethod;

FIG. 4 is a flowchart for explaining steps for determining the number ofbits for transmitting of each transmitting antenna and correspondingtransmitting power by using a equivalent channel gain according togreedy algorithm in accordance with a preferred embodiment;

FIG. 5 is a diagram showing an apparatus for adaptivelymodulating/demodulating signal in the MIMO wireless communication systemhaving layered space-time architecture in accordance with a preferredembodiment of the present invention;

FIG. 6 is a graph showing a performance of a method for adaptivelymodulating/demodulating signals in a MIMO wireless communication systemhaving V-BLAST type detector in accordance with a preferred embodimentof the present invention;

FIG. 7 is a diagram showing an apparatus for adaptivelymodulating/demodulating signals in MIMO OFDM wireless communicationsystem having V-BLAST type detector in accordance with a preferredembodiment of the present invention; and

FIG. 8 is a graph showing a result of a simulation of a method foradaptively modulating/demodulating signal in MIMO OFDM wireless systemhaving V-BLAST type detector in accordance with a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Other objects and aspects of the invention will become apparent from thefollowing description of the embodiments with reference to theaccompanying drawings, which is set forth hereinafter.

For helping to understand the present invention, a greedy algorithm usedin an adaptive modulation/demodulation method in accordance with thepresent invention is explained at first.

For explaining the greedy algorithm, an orthogonal frequency divisionmultiplexing method is used as an example.

At first, the transmission power of the n^(th) subcarrier of, P_(n), isgiven as:

$\begin{matrix}{P_{n} = \frac{f( C_{n} )}{{h_{n}}^{2}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

In the Eq. 1, h_(n) is a channel gain of the n^(th) subcarrier, C_(n) isthe number of bit for transmitting with the subcarrier, and f(C_(n)) isa is a transmission power for transmitting c bits with desired bit errorrate (BER).

In a meantime, if an entire bits for transmitting one of OFDM symbol isR, a bit allocation method for minimizing a sum of transmission power ofentire subcarrier can be expressed as:

$\begin{matrix}{P_{t} = {\min\limits_{c_{x} \in D}{\sum\limits_{n = 1}^{N_{c}}\;{\frac{1}{{h_{n}}^{2}}{f( C_{n} )}}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$subject to

$R = {\sum\limits_{n = 1}^{N_{c}}c_{n}}$

In the Eq. 2, Nc is the number of total subcarriers, D represents a setof information bits per subcarrier determined by modulation/demodulationmethods. For example, if a QPSK, 16-QAM, 64-QAM and so on can beselected for the modulation/demodulation methods, then D={0, 2, 4, 6, 8,. . . }. In here, 0 means that no information is sent in thecorresponding subcarrier.

The Eq. 2 can be solved by the greedy algorithm through following steps.

-   step 1: For all subcarriers, we define c_(n)=0,

${\Delta P}_{n} = {\frac{f({\Delta B})}{{h_{n}}^{2}}.}$

-   step 2: following equations 3, 4 and 5 are repeatedly operated until    the condition

$R = {\sum\limits_{n = 1}^{N_{c}}c_{n}}$is satified.

$\begin{matrix}{\hat{n} = {\arg{\min\limits_{n}P_{n}}}} & {{Eq}.\mspace{14mu} 3} \\{C_{n} = {C_{n} + {\Delta B}}} & {{Eq}.\mspace{14mu} 4} \\{{\Delta P}_{n} = \frac{{f( {C_{n} + {\Delta B}} )} - {f( C_{n} )}}{{h_{n}}^{2}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

In the Eqs. 4 and 5, Δβ is the difference of elements of set D e.g.,D={0, 2, 4, 6, . . . }. Therefore, Δβ=2.

step 3: after completing step 2, each C_(n) is determined and P_(n) iscalculated by using eq. 1.

Finally, the number of bits allocated to each subcarrier, C_(n), and thetransmission power, P_(n), is determined.

In the V-BLAST detection method, data transmitted from a plurality oftransmitting antenna is detected as one by one and the detected symbolsare cancelled for reducing interference to other signals. The nextsymbol is detected after canceling the channel effect corresponding tothe previously detected symbol. Therefore, the equivalent channel gainis varied according to an order of nulling. The present inventionprovides a method for achieving performance gain by adaptivelymodulating the signal with an equivalent channel gain obtained by thereverse order of conventional V-BLAST detection order. Hereinafter, theV-BLAST detection method is explained at first.

FIG. 1 is diagram for illustrating a MIMQ wireless communication systemhaving a layered space-time architecture detector in accordance with apreferred embodiment of the present invention.

Referring to FIG. 1, in the MIMO wireless communication system having adetector of V-BLAST structure, a signal vector transmitted from eachtransmission antenna 13-1, 13-2, . . . , 13-M is defined as x=[x₀, x₁, .. . , x_(M-1)]^(T) and a signal vector received to each receivingantenna 15-1, 15-2, . . . 15-N is defined as y=[y₀, y₁, . . . ,y_(N-1)]^(T). M and N is the number of transmitting/receiving antennasand it is satisfied with a condition, N≧M. The received signal vector isexpressed as:y=Hx+v  Eq. 6In the Eq. 6, H is N×M channel matrix, the element of the n^(th) rowand, a m^(th) column, h_(n,m), is a flat fading channel gain betweenm^(th) transmitting antenna and n^(th) receiving antenna. In a meantime,v is a N×1 white noise vector with zero mean and its covariance matrixis E[vv^(H)]=σ²I. I is N×N unit matrix.

For detection of the transmitting signal vector x from the receivingsignal vector y, the V-BLAST detection method is used. A nulling vectorfor k^(th) layer (k^(th) transmission antenna signal) is defined as:

$\begin{matrix}{{w_{k}^{T}(H)}_{l} = {\delta_{kl} = \{ \begin{matrix}{1,} & {k = l} \\{0,} & {k \neq l}\end{matrix} }} & {{Eq}.\mspace{14mu} 7}\end{matrix}$In the Eq. 7, (H)_(t) is 1^(th) column of matrix H and W_(k) ^(T) is ak^(th) column of a pseudo-inverse matrix H⁺. A detection order isdetermined by considering a value of w_(k). In the conventional V-BLASTdetection, a column of the matrix having smallest value of ∥w_(k)∥ isdetected at first. In a meantime, Z_(k) is a decision statistic ofk^(th) layer and it is expressed as:Z_(k)=x_(k)+w_(k) ^(T)v  Eq. 8

In the Eq. 8, k={1, 2, . . . , M}. After determining the signal byconsidering the decision statistic Z_(k), an interference of theprevious detected signal is cancelled and the channel matrix H isupdated. That is, new channel matrix H is generated by setting k^(th)column of prior matrix H as ‘0’. New received signal vector y′ isdetermined as:y′=y−(H)_(k)x_(k)  Eq. 9

In the Eq. 9, x_(k) is a result of decision of Z_(k). The Eqs. 6 to 9are repeatedly executed for calculating a nulling vector by using newchannel matrix H′ and receiving signal vector y′ until all signals aredetected. The above mentioned steps are detection steps of conventionalV-BLAST. Order of detection is determined according to the magnitude ofnorm of nulling vector.

If the eq. 7 is used for calculating a signal-to-noise ratio of thedecision statistics, ρ_(k), which is calculated as:

$\begin{matrix}{\rho_{k} = \frac{E\{ {x_{k}}^{2} \}}{\sigma^{2}{w_{k}}^{2}}} & {{Eq}.\mspace{14mu} 10}\end{matrix}$

In the, Eq. 10, E{ } is an expectation function and ∥w_(k)∥² is theequivalent channel gain. Therefore, a state of channel is better as∥_(k)∥ is getting smaller. The V-BLAST method for MIMO system using afixed modulation/demodulation method detects a layer of best channelstate at first and then detects a layer of worse channel state aftereliminating interference of signals, which have already been detected inorder to enhance performance of all layers.

However, the adaptive modulation/demodulation method detects a layer ofbest channel state later and information of a layer of worse channel isnot transmitted or less amount of information is transmitted in order toenhance the performance. Therefore, the detection order must to bereversed for enhancing the performance in accordance with the presentinvention.

FIG. 5 is a diagram showing an adaptive modulation/demodulation, and inthe MIMO wireless communication system with V-BLAST type in accordancewith a preferred embodiment of the present invention.

Referring to FIG. 5, a MIMO system having V-BLAST detector includes anadaptive modulation unit 52 and an adaptive demodulation unit 57 formodulating/demodulating signals with using differentdemodulation/modulation method and controlling transmission power of thesignal. The demodulation unit 57 includes a V-BLAST type detection unit.

Operations of the adaptive modulation/demodulation units 52 and 57 areexplained in detail as followings.

A transmitting data 51 is data transmitted from a transmitter. Ifnecessary, this date would be scrambled, channel coded, and interleaved.The adaptive modulation unit receives the channel information andcontrols transmitting power by modulating the data with differentmodulation methods according to each layer by using bit and powerallocation information.

When adaptively modulated signals are transmitted through M transmittingantennas 53-1 to 53-M, a bit and power allocation information calculator54 determines the number of bits and transmitting power to betransmitted to each transmitting antennas 53-1 to 53-M by using MIMOchannel information feedbacked from a receivers and the greedyalgorithm.

Through the above mentioned operations, signals transmitted from the Mtransmitting antennas 53-1 to 53-M are transmitted to N receivingantennas 56-1 to 56-N through frequency non-selective fading MIMOchannel. And the adaptive demodulation unit 57 demodulates the receivedsignals by using channel estimation result from a MIMO channel estimator58 and a bit allocate information from the bit allocation informationcalculator 59 in reverse order of V-BLAST. Finally, the demodulatedsignal is outputted.

FIG. 2 is a view showing changing of equivalent channel gain accordingto an order of nulling in a layered space-time architecture inaccordance with the present invention.

Referring to FIG. 2, although identical channel matrix is used, theequivalent channel gain is changed according to an order of detectionsuch as an order of V-BLAST, a random order and a reverse order ofV-BLAST. Thus, in case of detection according to the order of V-BLAST, adeviation of the equivalent channel gain is small and the sum ofequivalent channel gain is small. And in case of detecting according tothe reverse order of V-BLAST, the deviation and sum of the equivalentchannel gain become larger.

A method for determining the equivalent channel gain in a reverse orderof the V-BLAST detection method in accordance with a preferredembodiment of the present invention is explained in detail.

FIG. 3 is a flowchart for explaining the method for determining theequivalent channel gain in a reverse order of the V-BLAST detectionmethod.

Referring to FIG. 3, at step 31, an antenna index set is defined (S={1,2, . . . , M}) because of determining an order of detection fordetermining the equivalent channel gain. An pseudo-inverse Matrix ofchannel matrix H⁺ is calculated at step 32. At step 33, all nullingvectors w_(k) ^(T), kεS, are calculated based on the Eq. 7 forcalculating transmitting signal vector x.

After the step 33, square root of a norm of nulling vector is calculatedfor determining a layer for nulling and k^(th) layer having the largestvalue is selected at step 34. A column of the channel matrixcorresponding to the selected k^(th) layer is transformed to 0 vectorand a value k is eliminated from the set S at step 35. The abovementioned steps are repeated M times with newly transformed set S andchannel matrix H.

As mentioned above, an order of the selected layers becomes a detectionorder and an inverse of square root norm of the nulling vector for theselected layer becomes the equivalent channel gain. Furthermore, thenumber of bits transmitted at each layer (transmitting antenna) andcorresponding transmitting power is calculated by using the calculatedequivalent channel gain instead of subcarrier channel gain ∥h_(n)∥¹² ingreedy algorithm.

FIG. 4 is a flowchart for explaining steps for determining the number ofbits for transmitting of each transmitting antenna and correspondingtransmitting power by using a equivalent channel gain according togreedy algorithm in accordance with a preferred embodiment of thepresent invention.

Referring to FIG. 4, at step 41, the number of bits transmitted at eachlayer (each transmitting antenna) is initialized as ‘0’. Δβ is definedbased on the Eq. 2 and a transmitting power is calculated for obtainingan additional bit value of Δβ with desired bit error rate at each layerby using the equivalent channel gain of each layer. Among the calculatedlayers, a layer requiring the least transmitting power is selected andthe value of Δβ is putted to the selected layer. The above mentionedsteps are repeatedly performed until allocated total bit number isassigned. After determining the number of bits transmitted through eachlayer, a transmitting power of each layer is calculated based on the Eq.1.

FIG. 6 is a graph showing a performance of an adaptivemodulation/demodulation method of a MIMO wireless communication systemwith V-BLAST type detector in accordance with a preferred embodiment ofthe present invention. The graph shows result of simulation forcomparing performance of a conventional modulation/demodulation methodand the modulation/demodulation method in accordance with the presentinvention.

In the simulation, the number of transmitting and receiving antennas is4 and a channel of each transmitting and receiving antenna is a complexGaussian random variable with zero mean. For preventing to limitperformance by influence of specific channel matrix, more than 1000statistic channels are generated. The number of information bits fortransmitting in a predetermined time is set to 8 and QPSK and 16-QAM areused for modulation method. Thus, D={0, 2, 4} and therefore, Δβ is 2.

In the simulation, a system performance of a conventionalmodulation/demodulation method implemented by using greed algorithm withsingular value decomposition (SVD), which is known as the optimalsolution in the adaptive modulation MIMO system is used for a lowerbound for comparison. The conventional modulation/demodulation method isintroduced by G. G Raleigh et. al., in “spatio-temporal coding forwireless communication” at Proc. IEEE Globecom, November, 1996, pp.1809-1814 and it has too complicated structure. Thus, it is impossibleto be implemented to real system.

Referring to the FIG. 6, SVD is a result of the conventionalmodulation/demodulation method, BLAST is a result of using V-BLASTdetection order introduced by Ka-wai Ng and BLAST(random order) is aresult of using random detection order. BLAST(reverse) is a result ofusing reverse order of the V-BLAST detection order.

As shown in FIG. 6, the present invention has 0.7 bB performance gainand 1 dB performance loss in comparison with Ka-wai Ng's method and theconventional modulation/demodulation method proposed by Raleigh.

FIG. 7 is a diagram showing an adaptive modulation/demodulationapparatus of MIMO OFDM wireless communication system with V-BLAST typedetector in accordance with a preferred embodiment of the presentinvention.

Referring to FIG. 7, an adaptive modulation MIMO system of the presentinvention can be expanded to the MIMO OFDM system. It is because achannel matrix H of the MIMO system can be replaced with a channelmatrix in each subcarrier of MIMO-OFDM system. There are two methodsimplemented to the MIMO OFDM system.

A first method is a method for implementing adaptive modulation methodto all layers M and all subcarriers N_(c). At first, an equivalentchannel gain is independently calculated according to a layer of eachtransmitting antenna and subcarrier. The equivalent channel gain iscalculated according to the method in FIG. 4 as much as the number oflayers of the transmitting antenna times the number of subcarriers.Finally, N_(c)×R number of information bits and transmitting power isallocated according to the greedy algorithm introduced in FIG. 4 byusing the obtained N_(c)×R number of equivalent channel gains.

Therefore, when total data rate is determined in the MIMO-OFDMtransmitter, the number of information bits and correspondingtransmitting power can be determined by using the equivalent channelgain obtained by the greedy algorithm in FIG. 4 for allocating data bitsto total antennas and subcarriers. Inhere, a value of M in FIG. 4 is avalue of N becomes N_(c)×M. However, the first method is verycomplicated to be implemented in real system because of large number ofcomparison and equivalent channel gain calculation in the greedyalgorithm.

A second method is a method for adaptively modulating signal per eachsubcarrier, independently. That is, if total data rate is predeterminedin MIMO-OFDM system and the identical data bits are allocated to eachsub carrier wave, the data bits must be allocated by obtaining theequivalent channel gain of each subcarrier. In this case, the number ofthe equivalent channel gain is M per each subcarrier. The data bits areallocated by using the greedy algorithm in FIG. 4 in M layers. Thus, Nctimes of greedy algorithm are performed and the repetition number isdecreased. The second method can be simply implemented comparing to thefirst method but its performance is decreased compared with the firstmethod.

The bit and power allocation information calculator 73 allocates thedata bits by using the channel matrix H of each subcarrier delivered bythe MIMO channel estimation unit 82 and the transmission power is alsoallocated to each transmitting antennas 76-1 to 76-M by using theequivalent channel gain of each subcarrier.

The adaptive modulation unit based on V-BLAST type detector performsmodulation operations using bits and power allocated to eachtransmitting antenna 76-1 to 76-M per each subcarrier.

For demodulating a received signal at the MIMO OFDM system, bits andtransmitting power is known. Therefore, in the MIMO channel estimator 82at a receiver, a channel matrix H of each subcarrier is estimated andperforms operations for obtaining bits and power information using thegreedy algorithm. The V-BLAST detection and adaptive demodulation unit81 demodulates a signal using bit allocation information.

FIG. 8 is a graph showing a result of a simulation of an adaptivemodulation/demodulation method of MIMO OFDM wireless system with V-BLASTtype detector in accordance with a preferred embodiment of the presentinvention.

Referring to FIG. 8, the simulation is progressed by using 4transmitting and receiving antennas, 64 subcarriers and 16 cyclicprefixes are used for OFDM modulation. An exponentially decayingquasi-static fading channel having 8 taps is used.

The method as mentioned above can be implemented as a program and can bestored in a computer readable recording medium such as CD-ROM, RAM, ROM,Floppy disk. Hard disk and optical magnetic disk.

The present invention can improve performance above 0.7˜1.4dBperformance gain comparing to conventional modulation method with randomand V-BLAST detection orders without increase of implementationcomplexity.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. An apparatus for adaptively modulating signal in a MIMO system havinga layered space-time architecture based detector, the apparatuscomprising: a bit and power allocation information calculator fordeciding an equivalent channel gain in a reverse order of Vertical-Belllaboratories Space Time (V-BLAST) based on MIMO channel informationfeedbacked from a receiver and determining the number of bits andtransmission power to be transmitted to each transmitting antenna byusing the equivalent channel gain; and adaptive modulation mean formodulating signal of each layer with corresponding modulation methodbased on the determined number of bits, controlling the transmittingpower and transmitting the adaptively modulated signal through eachtransmitting antenna.
 2. The apparatus as recited in claim 1, whereinthe transmitter detects and modulates signals in a reverse order of aV-BLAST.
 3. The apparatus as recited in claim 1, wherein the receivertransmits identical adaptive modulation information with the modulationmethod and transmitting power instead of feedbacking the MIMO channelinformation.
 4. The apparatus as recited in claim 1, wherein the bit andpower allocation information calculation mean determines an equivalentchannel gain in the reverse order of V-BLAST and calculates the numberof bit information and corresponding transmitting power at each layer byusing the equivalent channel gain in a greedy algorithm instead of usingchannel gain.
 5. An apparatus for adaptively demodulating signal in aMIMQ system having a layered space-time architecture based detector, theapparatus comprising: MIMO channel estimation means for estimating MIMOchannel from a signal received through each receiving antenna; a bit andpower allocation information calculator for determining an equivalentchannel gain in reverse order of Vertical-Bell laboratories Space Time(V-BLAST) based on MIMO channel information from the MIMO channelestimation means and determining the number of bits to be transmittedfrom each transmitting antenna by using the equivalent channel gain; andadaptive demodulation means for demodulating signal of each layer withcorresponding modulation method based on the determined number of bitsand the MIMO channel information.
 6. An apparatus for adaptivelymodulating and demodulating signals in MIMO system using multipleantennas at transmitter and receivers, the apparatus comprising: anadaptive modulation means for adaptively modulating signals in order totransmit the modulated signal after determining an equivalent channelgain in a reverse order of a vertical-bell laboratories space time anddetermining the number of bits and transmitting power based on thedetermined equivalent channel gain; and adaptive demodulation means fordetecting and adaptively demodulating received signals in reverse orderof V-BLAST.
 7. The apparatus as recited in claim 6, wherein the adaptivedemodulation means feedbacks the MIMO channel information to themodulation means or transmits identical adaptive modulation informationincluding the modulation method and transmitting power instead of theMIMO channel information.
 8. The apparatus as recited in claim 7,wherein the apparatus has (the number of subcarriers).times.(the numberof transmission antennas) of equivalent channel gains and determines thenumber of bits and transmitting power to be transmitted through eachtransmission antennas in a MIMO-OFDM system having a layered space-timearchitecture detector.
 9. The apparatus as recited in claim 6, whereinthe apparatus independently detects and demodulates signals per eachsubcarrier by determining the number of bits and transmitting power tobe transmitted through each transmitting antenna per each subcarrier ina MIMO OFDM system having a layered space-time architecture detector.10. A method for adaptive modulating signals in a MIMO system usingmultiple antennas in a receiver and transmitter, the method comprisingthe steps of: a) determining equivalent channel gain in a reverse orderof V-BLAST at transmitter based on information feedbacked from thereceivers; and b) adaptively modulating signals by determining thenumber of bits and corresponding transmitting power to be transmittedthrough each layer by using the equivalent channel gain in a greedyalgorithm instead of using subcarrier.
 11. The method as recited inclaim 10, wherein the transmitter detects and modulates signals in areverse order of a V-BLAST.
 12. The method as recited in claim 11,wherein the step a) includes the steps of: a-1) initializing an antennaindex set; a-2) nulling layers in an order of layer having a smallestequivalent channel gain and modifying the antenna index set or a channelmatrix based on a result of nulling layers; a-3) repeatedly performingthe step a-2) as many as the number of antennas; and a-4) deciding theequivalent channel gain of each antenna layer based on a result of thestep a-3).
 13. The method as recited in the claim 12, wherein the stepa-2) includes the steps of: a-2-1) computing an pseudo-inverse matrix ofchannel matrix; a-2-2) computing square root of norm of nulling vectorfor deciding a nulling layer; a-2-3) selecting a layer having a biggestnorm of nulling vectors among layers of computing results from the stepa-2-2); and a-2-4) nulling the selected layer and eliminating theselected layer from the antenna index set.
 14. The method as recited inclaim 10, the step a) includes the steps of: a-a) initializing anantenna index set; a-b) nulling layers in an order of layer having asmallest equivalent channel gain and modifying the antenna index set ora channel matrix based on a result of nulling layers; a-c) reputedlyperforming the step a-b) as many as the number of antennas; a-d)deciding the equivalent channel gain of each antenna layer based on aresult of the step a-c); and a-e) deciding the equivalent channel gainby reputedly performing the step a), the step b), the step a-a) and thestep a-c).
 15. The method as recited in the claim 14, the number of bittransmitting to each antenna and a transmitting power are decided byperforming a greedy algorithm based on the decided equivalent channelgain computed from the step a-e) in the step b).
 16. The method asrecited in claim 10, wherein the step a), in a case there are presetdata bits of the system and data bits allocated to each subcarrier areidentical, includes the steps of: a-I) initializing an antenna index setaccording to a subcarrier; a-II) nulling layers in an order of layerhaving a smallest equivalent channel gain and modifying the antennaindex set or a channel matrix based on a result of nulling layers forthe subcarrier; a-III) repeatedly performing the step a-II) as many asthe number of antennas for the subcarrier; and a-IV) deciding theequivalent channel gain of each antenna layer based on a result of thestep a-III).
 17. The method as recited in claim 16, wherein the stepa-II) includes the steps of: a-II-1) performing a greedy algorithm fromthe equivalent channel gain decided to one of the subcarrier in the stepof a-IV) and deciding the number of bits transmitting through an antennaaccording to the one of carrier waves and transmitting power; anda-II-2) deciding identical number of bits and transmitting power whichare decided in the step a-II-1) for all other sub carrier waves.
 18. Amethod for adaptively demodulating in a multi input and multi outputsystem, the method comprising the steps of: a) estimating a channel froma signal received at each receiving antenna; b) deciding an equivalentchannel gain in a reverse order of a vertical-bell laboratories spacetime (V-BLAST) based on the channel information; and c) detecting andadaptively demodulating by deciding the number of bits based on theequivalent channel gain.
 19. A computer readable recoding medium storinginstruction for executing a method for adaptive modulation, the methodcomprising the steps of: a) at a transmitter, deciding an equivalentchannel gain in a reverse order of V-BLAST based on a feedbackinformation from a receiver; and b) at the transmitter, deciding thenumber of bit transmitting through each layer (transmitting antenna) andtransmitting power based on the equivalent channel gain computed at thestep a).
 20. A computer readable recoding medium storing instructionsfor executing a method for adaptively demodulating signals, the methodcomprising the steps of: a) estimating a channel from a signal receivedfrom each receiving antenna; b) deciding an equivalent channel gain in areverse order of V-BLAST based on the channel information; and c)detecting and adaptively demodulating by deciding the number bitstransmitted from the each transmitting antenna based on the equivalentchannel gain.
 21. An apparatus for adaptively modulating signal in aMIMO system, the apparatus comprising: a bit and power allocationinformation calculator configured to determine an equivalent channelgain for each trasmitting antenna in a reverse order of a Vertical-BellLaboratories Space Time (V-BLAST) and determine the number of bits andtransmission power for each transmitting antenna by using the equivalentchannel gain; and an adaptive modulator configured to modulate a signalto be transmitted through each transmitting antenna based on thedetermined number of bits and transmit the modulated signal through eachtransmitting antenna at the determined transmission power.
 22. Theapparatus as recited in claim 21, wherein the bit and power allocationinformation calculator configured to determine the equivalent channelgain based on MIMO channel information feedbacked from a receiver. 23.An apparatus for adaptively demodulating signal in a MIMO system, theapparatus comprising: a MIMO channel estimator configured to estimate aMIMO channel from a signal received through each receiving antenna; abit and power allocation information calculator configured to determinean equivalent channel gain for each receiving antenna in reverse orderof a Vertical-Bell Laboratories Space Time (V-BLAST) and determine thenumber of bits for each receiving antenna by using the equivalentchannel gain; and an adaptive demodulator configured to demodulate thesignal of each layer based on the determined number of bits.
 24. Theapparatus as recited in claim 23, wherein the bit and power allocationinformation calculator configured to determine the equivalent channelgain based on MIMO channel information from each transmitting antenna.25. A system for adaptively modulating and demodulating signals in MIMOsystem, the system comprising: an adaptive modulator configured tomodulate signals to be transmitted through each transmitting antennaafter determining an equivalent channel gain in a reverse order of aVertical- Bell Laboratories Space Time (V-BLAST) and determine thenumber of bits based on the determined equivalent channel gain; and anadaptive demodulator configured to detect and demodulate receivedsignals in reverse order of V-BLAST.
 26. A method for adaptivelymodulating signals in a MIMO system, the method comprising the steps of:determining an equivalent channel gain for each transmitting antenna ina reverse order of a Vertical-Bell Laboratories Space Time (V-BLAST);and adaptively modulating signals by determining the number of bits tobe transmitted through each transmitting antenna based on the equivalentchannel gain in a greedy algorithm.
 27. The method as recited in claim26, wherein the equivalent channel gain is determined based on MIMOchannel information feedbacked from a receiver.
 28. The method asrecited in claim 26, wherein the step of determining the equivalentchannel gain comprises steps of: initializing an antenna index set;nulling layers in an order of layer having a smallest equivalent channelgain and modifying the antenna index set or a channel matrix based on aresult of nulling layers; and deciding the equivalent channel gain ofeach antenna layer based on a result of nulling.
 29. The method asrecited in the claim 28, wherein the step of nulling comprises steps of:computing an pseudo-inverse matrix of channel matrix; computing squareroot of norm of nulling vector for deciding a nulling layer; selecting alayer having a biggest norm of nulling vectors among layers of computingresults from the step of computing square root of norm of nulling vectorfor deciding a nulling laye; and nulling the selected layer andeliminating the selected layer from the antenna index set.
 30. A methodfor adaptively demodulating in a MIMO system, the method comprising thesteps of: estimating a channel from a signal received through eachreceiving antenna; determining an equivalent channel gain for eachreceiving antenna in a reverse order of a Vertical-Bell LaboratoriesSpace Time (V-BLAST); and adaptively demodulating the received signalsby deciding the number of bits based on the equivalent channel gain. 31.The method as recited in claim 30, wherein the equivalent channel gainis determined based on MIMO channel information.
 32. A computer readablerecoding medium storing instruction for executing a method for adaptivemodulation, the method comprising the steps of: determining anequivalent channel gain for each transmitting antenna in a reverse orderof V-BLAST; and determining a number of bit to be transmitted througheach transmitted antenna based on the equivalent channel gain.
 33. Thecomputer readable recoding medium as recited in claim 32, wherein theequivalent channel gain is determined based on on MIMO channelinformation feedbacked from a receiver.
 34. A computer readable recodingmedium storing instructions for executing a method for adaptivelydemodulating signals, the method comprising the steps of: estimating achannel from a signal received through each receiving antenna;determining an equivalent channel gain for each receiving antenna in areverse order of V-BLAST; and adaptively demodulating the receivingsignals by deciding number of bits based on the equivalent channel gain.35. The computer readable recoding medium as recited in claim 34,wherein the equivalent channel gain based on MIMO channel informationfrom each transmitting antenna.